1. Field of the Invention
The present disclosure generally relates to an image forming apparatus having a neutralization unit configured to remove electric charge from a recording medium (e.g., transfer sheet), which has just passed through a transfer nip.
2. Description of the Related Art
In an image forming apparatus, toner images on an image carrying member can be directly transferred to a recording medium (e.g., transfer sheet) by a transfer unit, or toner images on the image carrying member can be transferred to an intermediate transfer member and then transferred to the recording medium.
In such an image forming apparatus, toner images are transferred to the recording medium at a transfer nip, formed between the transfer unit and the image carrying member (or intermediate transfer member), when the recording medium passes through the transfer nip. The transfer unit and the image carrying member (or intermediate transfer member) generate a transfer electric field at the transfer nip.
The transfer unit applies a transfer bias voltage to a back face of the recording medium (e.g., transfer sheet), wherein such transfer bias voltage has a relatively large voltage and a polarity, which is opposite to a polarity of toner images, on the image carrying member (or intermediate transfer member).
With such transfer bias voltage, toner images can be transferred to a front face of the recording medium from the image carrying member (or intermediate transfer member).
Because such transfer bias voltage is applied to the back face of the recording medium, the back face of the recording medium is charged with a polarity, which is same as the transfer bias voltage when the recording medium passes through the transfer nip. In other words, the back face of the recording medium is charged with polarity, which is opposite to a polarity of toner images on the image carrying member (or intermediate transfer member).
Such an electric charge on the back face of the recording medium may be used to retain toner images on the front face of the recording medium.
However, if the back face of the recording medium, which has just passed through the transfer nip, has too much electric charge thereon, an electrostatic adsorbability of the recording medium to the image carrying member may become too large, by which the recording medium may not be effectively separated from the image carrying member (or intermediate transfer member), and a sheet jamming may occur.
Furthermore, if the back face of the recording medium has too much electric charge, a sudden electric leak may occur from the back face of recording medium to parts (e.g., metal parts) provided around a transport path between the transfer nip and a fixing unit, by which toner images on the recording medium may be disturbed, and tiny circle-like patterns may occur on the toner images on the recording medium.
Furthermore, if the back face of the recording medium has too much electric charge, the front face of the recording medium may develop an electric charge, which has a polarity opposite to the polarity on the back face of the recording medium.
In such a condition, the electric charge on the front face of the recording medium may flow along the surface of the recording medium when the recording medium is transported in the transport path between the transfer nip and the fixing unit, by which toner images on the recording medium may be disturbed, and a lightning bolt pattern, which may correspond to a electric charge flow, may occur on the toner images on the front face of the recording medium.
In order to suppress such drawbacks, an image forming apparatus can include a neutralization unit, at a near portion of an exit of the transfer nip, to remove electric charge from a back face of a recording medium (e.g., transfer sheet) after the recording medium passes through the transfer nip.
The neutralization unit may include an electric-charge removing member, made of electric conductive material, and have a plurality of exposed areas along a longitudinal direction of the electric-charge removing member. The longitudinal direction of the electric-charge removing member may be arranged perpendicular to a transport direction of the recording medium. The plurality of exposed areas of the electric-charge removing member may be arranged closely to the back face of the recording medium.
With such a neutralization unit, excessive charge on the back face of the recording medium, which has passed through the transfer nip, can be removed, and thereby the above-mentioned drawbacks may be prevented.
In order to efficiently remove electric charge from the recording medium by such an electric-charge removing member, the plurality of exposed areas of the electric-charge removing member is preferably placed as close to the back face of the recording medium as possible, wherein the electric-charge removing member may be provided near the exit of the transfer nip.
Specifically, the plurality of exposed areas of the electric-charge removing member is placed in proximity to a nip tangent line of the transfer nip, wherein the nip tangent line is a tangent line extended from the transfer nip in a transport direction of the recording medium.
However, if the electric-charge removing member is arranged in such manner, the back face of the recording medium (e.g., transfer sheet), which has just passed through the transfer nip, may directly contact the electric-charge removing member because the recording medium may not be transported along the nip tangent line, but may sometimes be transported in a direction deviated from the nip tangent line.
If the back face of the recording medium contacts the electric-charge removing member, the electric charge on the back face of the recording medium may suddenly leak to the electric-charge removing member, and may result in the occurrence of an abnormal image (ie., an image that includes tiny circle-like patterns).
In order to prevent such contact between the recording medium and the electric-charge removing member, the neutralization unit may further include a plurality of ribs, which protrude from a surface of the plurality of exposed areas of the electric-charge removing member, wherein such ribs face toward the back face of the recording medium (e.g., transfer sheet).
FIG. 1A shows a schematic view for explaining a conventional neutralization unit provided near the transfer nip, and FIG. 1B shows an expanded view of the neutralization unit in FIG. 1A.
As shown in FIG. 1A, a recording medium S (e.g., transfer sheet) passes through a transfer nip, formed between an image carrying member (e.g., photoconductive member I) and a transfer unit (e.g., transfer roller 21).
The transfer roller 21, applied with a transfer bias voltage, can transfer toner images from the photoconductive member I to a front face of the recording medium S.
As shown in FIG. 1A, a neutralization unit 340 is provided in a downstream of a transport direction of the recording medium S and near the transfer nip.
As shown in FIG. 1B, the neutralization unit 340 includes an insulating support member 341, an electric-charge removing member 343, and a rib 342.
The electric-charge removing member 343, made of electrically conductive material, may be fixed on the insulating support member 341. The electric-charge removing member 343 is applied with an electric charge removing bias voltage, which has a polarity the same as toner images, from a power source (not shown), to remove electric charge from the back face of the recording medium S.
As shown in FIG. 1B, a plurality of exposed areas 343a are provided along a longitudinal direction B of the electric-charge removing member 343.
The longitudinal direction B of the neutralization unit 340 can be arranged in a direction perpendicular to a transport direction A of the recording medium S, and the exposed areas 343a can be placed in proximity to the back face of the recording medium S, which has just passed through the transfer nip.
As shown in FIG. 1B, the neutralization unit 340 includes a plurality of ribs 342, which may be made of insulating material.
Each of the ribs 342 may be integrally formed with the insulating support member 341 and each of the ribs 342 may be provided between adjacent exposed areas 343a as shown in FIG. 1B. Such ribs 342 are projected from each of the exposed areas 343a toward the back face of the recording medium S.
With such configuration, the back face of the recording medium S, which has just passed through the transfer nip, may not contact the electric-charge removing member 343 because the recording medium S may contact the ribs 342. In other words, the rib 342 prevents the back face of the recording medium S contacting the electric-charge removing member 343.
With such neutralization unit 340, a sudden electric-charge leaking from the back face of the recording medium S to the electric-charge removing member 343 may be prevented. Thus, an occurrence of an abnormal image, such as an image including tiny circle-like patterns, can be prevented.
However, if an image forming operation is conducted with the neutralization unit 340 having the ribs 342, streak lines may be produced on an image with a given interval, which corresponds to an interval of adjacent exposed areas 343a (or adjacent ribs 342), wherein streak lines may occur as an abnormal line image, extending in the transport direction A on the recording medium S.
The ribs 342 may cause such streak lines as discussed below. In general, streak lines may prominently appear on transfer sheets when printing a number of sheets continuously (e.g., at a time before completing continuous printing). During such printing, each of the ribs 342 may be charged by a friction with the back face of the recording medium S, and may accumulate electric charge, by which toner images on the front face of the recording medium S may be disturbed.
FIG. 2 shows a configuration for measuring the electric charge on the ribs 42.
As shown in FIG. 2, a surface electrometer can be connected to the insulating support member 341, and a value measured by the surface electrometer can be assumed as electric charge of the ribs 342. In one example measurement, the ribs 342 are charged to +3,000 to +4,000 V (voltage) when printing a number of sheets continuously (e.g., at a time before completing continuous printing), wherein such value is higher than a transfer bias voltage (e.g., +2,000 V).
Therefore, electric charge may be accumulated on the ribs 342 by a friction with the back face of the recording medium S, and the accumulated electric charge may disturb toner images on the front face of the recording medium S.
Such streak lines may be suppressed by reducing frictional electric charges formed on the ribs 342.
Making the ribs 342 with a material, which is hard to be charged by friction, can reduce the frictional electric charges on the ribs 342. However, an inexpensive insulating material such as ABS (acrylonitrile-butadiene-styrene) may be easily charged by friction, and a material hard to be charged by friction may unfavorably increase the manufacturing costs of the neutralization unit 340.
Similarly, the above-mentioned streak lines may occur when an image carrying member is applied with a transfer bias voltage, having a same polarity as that of the toner, to transfer toner images from the image carrying member to a recording medium (e.g., transfer sheet) at a transfer nip.
A shape of the ribs 342 may influence electric charge generated on the ribs 342 by a friction.
For example, the conventional rib 342 shown in FIG. 1B has a triangular shape when viewed from the longitudinal direction B of the electric-charge removing member 343, and one side of the triangular shaped rib 342 may extend in the transport direction A of the recording medium S.
Because the back face of the recording medium S, which has just passed through the transfer nip, may move along such one side of the ribs 342, the back face of the recording medium S may be frictioned with the one side of the ribs 342, which has a relatively larger area, and frictional electric charge generated one the rib 342 may become large.